The present invention relates to a process for the homopolymerization of vinylaromatic monomers or the copolymerization of vinylaromatic monomers and dienes in the presence of at least one alkali metal organyl, at least one magnesium organyl and at least one aluminum organyl and to an initiator composition for carrying out the process.
Anionic polymerizations typically proceed very rapidly, so that they are difficult to control on an industrial scale owing to the considerable amount of heat generated. Lowering the polymerization temperature results in an excessive increase in viscosity, in particular with a concentrated solution. Reducing the initiator concentration increases the molecular weight of the polymer formed. Controlling the reaction by appropriate dilution of the monomers results in a higher solvent requirement and lower space-time yields.
It has therefore been proposed to include in the anionic polymerization initiators various additives to influence the polymerization rate.
The effect of Lewis acids and Lewis bases on the rate of the anionic polymerization of styrene has been described in Welch, Journal of the American Chemical Society, Vol 82 (1960), pages 6000-6005. For instance, it has been found that small amounts of Lewis bases such as ethers and amines accelerate the n-butyllithium-initiated polymerization of styrene at 30xc2x0 C. in benzene, whereas Lewis acids such as zinc and aluminum alkyls reduce the polymerization rate or, when used in superstoichiometric amounts, stop the polymerization completely.
In Macromolecules, Vol 19 (1966), pages 299 to 304, Hsieh and Wang investigated the complexation of dibutylmagnesium with the alkyllithium initiator and/or with the living polymer chain in the presence and absence of tetrahydrofuran and found that dibutylmagnesium reduces the polymerization rate of styrene and butadiene without affecting the stereochemistry.
U.S. Pat. No. 3,716,495 discloses initiator compositions for the polymerization of conjugated dienes and vinylaromatics where a more efficient use of the lithium alkyl as initiator is achieved by the addition of a metal alkyl of a metal of group 2a, 2b or 3a of the Periodic Table of the Elements, such as diethyl zinc and polar compounds such as ethers or amines. Owing to the required large amounts of solvent, relatively low temperatures and long reaction times in the region of several hours, the space-time yields are correspondingly poor.
WO97/33923 describes initiator compositions which are used for the anionic polymerization of vinyl monomers and comprise alkali metal and magnesium compounds carrying hydrocarbon radicals and have a molar [Mg]/[alkali metal] ratio of at least 4.
PCT/EP97/04497, which was unpublished at the priority date of the present invention, describes continuous processes for the anionic polymerization or copolymerization of styrene or diene monomers using alkali metal alkyl as polymerization initiator in the presence of an at least bivalent element as a retarder.
Various initiator mixtures which may comprise alkali metals, alkaline earth metals, aluminum, zinc or rare earth metals are known, for example, from EP-A 0 234 512 for the polymerization of conjugated dienes with a high degree of 1,4-trans-linking. German Offenlegungsschrift 26 28 380 teaches, for example, the use of alkaline earth aluminates as cocatalyst in conjunction with an organolithium initiator for the preparation of polymers or copolymers of conjugated dienes having a high trans-1,4-linkage content and low 1,2-linkage or 3,4-linkage content. This is said to lead to an increase in polymerization rate.
Polydienes having a high proportion of 1,2-linking of the diene monomer units have been prepared using cyclic acetals of a glyoxal (U.S. Pat. Nos. 4,520,123, 4,591,624) or trisubstituted phosphine oxides (U.S. Pat. No. 4,530,984). In addition to the anionic initiators based on lithium, magnesium and/or aluminum alkyls can be used as coinitiators.
The use of additives such as aluminum alkyls which have a strong retarding effect on the anionic polymerization requires exact dosage and temperature control. A slight underdosage may lead to an insufficient retardation of the reaction rate, whereas a slight overdosage may completely stop the polymerization.
To achieve a sufficient retardation of the polymerization rate, weakly retarding additives such as magnesium dialkyls must be added in amounts which are significantly larger than the stoichiometric amount based on the alkali organyl initiator. The magnesium alkyls do not act as polymerization initiators on their own, but may initiate additional polymer chains in the presence of lithium organyls. The molecular weight of the polymers is therefore not only dependent on the molar ratio of alkali organyl initiator to monomer, but is also affected by the amount of magnesium organyl, the temperature and the concentration. In addition to higher costs, large amounts of retarding additives may also lead to altered product properties such as poor transparency, since the initiator components usually remain in the polymer.
It is an object of the present invention to provide a process for the homopolymerization of vinylaromatic monomers or the copolymerization of vinylaromatic monomers and dienes which does not have the abovementioned disadvantages, and, in particular, to provide an initiator composition for the process which makes it possible to adjust the polymerization rate within wide temperature and concentration ranges.
We have found that this object is achieved by a process for the homopolymerization of vinylaromatic monomers or the copolymerization of vinylaromatic monomers and dienes, which comprises polymerizing the monomers in the presence of at least one alkali metal organyl, at least one magnesium organyl and at least one aluminum organyl.
The invention also provides an initiator composition comprising at least one alkali metal organyl, at least one magnesium organyl and at least one aluminum organyl wherein
a) the molar ratio of magnesium to alkali metal is in the range from 0.2 to 3.8,
b) the molar ratio of aluminum to alkali metal is in the range from 0.2 to 4,
and a process for the preparation of an initiator composition, in which the metal organyls, dissolved in inert hydrocarbons, are mixed together and aged at a temperature in the range from 0 to 120xc2x0 C. for at least 5 minutes.
Alkali metal organyls which may be used are mono-, bi- or multifunctional alkali metal alkyls, aryls or aralkyls customarily used as anionic polymerization initiators. It is advantageous to use organolithium compounds such as ethyllithium, propyllithium, isopropyllithium, n-butyllithium, sec-butyllithium, tert-butyllithium, phenyllithium, diphenylhexyllithium, hexamethylenedilithium, butadienyllithium, isoprenyllithium, polystyryllithium or the multifunctional compounds 1,4-dilithiobutane, 1,4-dilithio-2-butene or 1,4-dilithiobenzene. The amount of alkali metal organyl required depends on the desired molecular weight, the type and amount of the other metal organyls used and the polymerization temperature and is typically in the range from 0.0001 to 5 mol percent, based on the total amount of monomers.
Magnesium organyls which may used are those of the formula R2Mg, wherein the radicals R are each, independently of one another, hydrogen, halogen, C-C20-alkyl or C6-C20-aryl. Preference is given to using the commercially available ethyl, propyl or butyl compounds. Particular preference is given to using (n-butyl)(s-butyl)magnesium which is soluble in hydrocarbons.
Aluminum organyls which may be used are those of the formula R3Al, wherein the radicals R are each, independently of one another, hydrogen, halogen, C1-C20-alkyl or C6-C20-aryl. Preferred aluminum organyls are aluminum trialkyls such as triethylaluminum, triisobutylaluminum, tri-n-butylaluminum, triisopropylaluminum, tri-n-hexylaluminum. Particular preference is given to using triisobutylaluminum. It is also possible to use aluminum organyls which are formed by partial or complete hydrolysis, alcoholysis, aminolysis or oxidation of alkyl- or arylaluminum compounds or those which carry alkoxide, thiolate, amide, imide or phosphide groups. Examples are diethylaluminum ethoxide, diisobutylaluminum ethoxide, diisobutyl-(2,6-di-tert-butyl-4-methyl-phenoxy)aluminum (CAS No. 56252-56-3), methylaluminoxane, isobutylated methylaluminoxane, isobutylaluminoxane, tetraisobutyldialuminoxane, bis(diisobutyl)aluminum oxide or diethylaluminum (N,N-dibutylamide).
The molar ratios of the metal organyls with respect to each other may vary within wide limits, but depend primarily on the desired retardation effect, the polymerization temperature, the monomer composition and concentration and the desired molecular weight.
The molar ratio of magnesium to alkali metal is advantageously in the range from 0.1 to 10, preferably in the range from 0.2 to 3.8, particularly preferably in the range from 1 to 3. The molar ratio of aluminum to alkali metal is in the range from 0.1 to 10, preferably in the range from 0.2 to 4, particularly preferably in the range from 0.7 to 2. The molar ratio of magnesium to aluminum is preferably in the range from 0.05 to 8.
In the process of the invention use is made primarily of alkali metal organyls, magnesium organyls and aluminum organyls. Barium, calcium or strontium organyls are preferably only present in ineffective amounts not having a significant effect on the polymerization rate or copolymerization parameters. Transition metals or lanthanoids, especially titanium and zirconium, should not be present in significant amounts.
The alkali metal, magnesium and aluminum organyls may be added to the monomer mixture together or separately and at different times or different locations. The alkali metal, magnesium and aluminum alkyls are preferably used in the form of a premixed initiator composition.
The initiator composition may be prepared by dissolving the alkali metal organyls, the magnesium organyls and the aluminum organyls in an inert hydrocarbon solvent, for example n-hexane, n-heptane, cyclohexane, ethylbenzene or toluene, and combining the solutions. The metal organyls dissolved in the hydrocarbons are preferably mixed together and aged at a temperature in the range from 0 to 120xc2x0 C. for at least 5 minutes. A solubilizer, for example diphenylethylene, can be added, if necessary, to prevent the precipitation of one of the components from this initiator solution.
The initiator solution is suitable for the polymerization of anionically polymerizable monomers. The initiator composition is preferably used for the homopolymerization or copolymerization of vinylaromatic monomers and dienes. Preferred monomers are styrene, xcex1-methylstyrene, p-methylstyrene, ethylstyrene, tert-butylstyrene, vinyltoluene or 1,1-diphenylethylene, butadiene, isoprene, 2,3-dimethylbutadiene, 1,3-pentadiene, 1,3-hexadiene or piperylene or mixtures thereof.
The polymerization may be carried out in the presence of a solvent. Suitable solvents are the aliphatic, cycloaliphatic or aromatic hydrocarbons having from 4 to 12 carbon atoms which are generally used for anionic polymerization, such as pentane, hexane, heptane, cyclohexane, methylcyclohexane, isooctane, decalin, benzene, alkylbenzenes such as toluene, xylene, ethylbenzene or cumene or suitable mixtures. Obviously, the solvent should have the high purity which is typically required for the process. The solvent may be dried over aluminum oxide or molecular sieve and/or distilled prior to use to remove protic substances. The solvent from the process is preferably reused after condensation and the abovementioned purification.
It is possible to adjust the retarding effect within wide temperature ranges via the composition and amount of the metal organyls. It is therefore also possible to carry out the polymerization at initial monomer concentrations in the range from 50 to 100 percent by volume, particularly from 70 to 100 percent by volume, which lead to highly viscous polymer solutions and require higher temperatures, at least at higher conversions.
After the polymerization is completed, the living polymer chains may be capped with a chain terminator. Suitable chain terminators are protic substances or Lewis acids, such as water, alcohols, aliphatic or aromatic carboxylic acids and inorganic acids such as carbonic acid or boric acid.
The target products may be homopolymers or copolymers and mixtures thereof. Polystyrene and styrene/butadiene block copolymers are preferably obtained. The process of the invention may also be used to prepare high-impact polystyrene (HIPS), in which case polybutadiene, styrene/butadiene block copolymers or mixtures thereof may be used as rubbers.
The block copolymers may be coupled using multifunctional compounds such as polyfunctional aldehydes, ketones, esters, anhydrides or epoxides.
The process of the invention may be carried out in any pressureand temperature-resistant reactor, it being possible in principle to use backmixing or non-backmixing reactors (i.e. reactors having stirred tank or tubular reactor characteristics). Depending on the choice of initiator concentration and composition, the particular process route applied and other parameters, such as temperature and possible temperature profile, the process of the present invention leads to polymers having high or low molecular weights. It is possible to use, for example, stirred tanks, tower reactors, tube reactors and tubular reactors or tube bundle reactors with or without internals. Internals may be static or mobile.
The process is preferably carried out continuously. It is preferred to carry out at least a part of the conversion, particularly conversions of between 50 and 100%, in a non-backmixing reactor or reactor section.
The initiator composition according to the invention makes it possible to significantly reduce the reaction rate or increase the temperature, respectively, without affecting the polymer properties compared to anionic polymerization using an alkali metal organyl; this makes it possible, on the one hand, to spread out the generation of the heat of polymerization over a longer period of time and thus control, in a continuous process, the temperature profile as a function of time or location, e.g. in a tubular reactor. It is possible, for example, to ensure that a high temperature does not occur at initially high monomer concentration, whereas, on the other hand, a trouble-free polymerization is possible at the high temperature which is finally (i.e. at higher conversion) reached while achieving a high space-time yield at the same time. In this process, fouling no longer occurs.